Static and dynamic activity of warm receptors in Boa constrictor

Summary

Afferent impulses from multi- and single-fiber preparations of the trigeminal nerve inBoa constrictor were recorded during exactly controlled thermal stimulation of the receptive field in the labial region. At constant temperatures in the range between 18 and 37°C, multi-fiber preparations showed a continuous discharge with a maximum around 30°C. Dynamic warming caused a high increase of the discharge, whereas dynamic cooling led to a complete inhibition. No cold-sensitive fibers have been found. Mechanical stimulation elicited large spikes from specific mechanoreceptors.

Single-fiber preparations from labial warm receptors did not respond to mechanical stimulation. Their discharge was always regular at constant temperatures. The average frequency of a warm receptor population was zero at about 18°C, reached a maximum of 13 sec−1 at 30°C and fell again to zero at 37°C. In addition, a few warm receptors increased their static discharge with temperature up to 36°C, the highest frequency being 38 sec−1. Stepwise warming by ΔT=+5°C caused a marked overshoot in frequency, after which the discharge usually fell to a minimum and then rose again to a new static level. Stepwise cooling by ΔT=−5°C led to a transient inhibition of activity followed by an increase until the new static level was reached. In the first group of warm receptors the height of the dynamic overshoot varied with the adapting temperature, the largest average overshoot of 160 sec−1 occurring at an adapting temperature of 30°C. These receptors have their static maximum as well as their highest dynamic sensitivity in the temperature range of the natural tropical habitat of Boidae.